ABSTRACT
Solid-liquid extraction is one of the unit operations of process engineering, the objective of which is to effect the migration of a substance enclosed in a solid insoluble matrix to a surrounding solvent, analogous to a desorption. In the case of sugar beet extraction (for sugar cane extraction see Secs. X-XV, and for starch extraction see Sec. XVI and XVII), sucrose is present in the form of an aqueous solution (juice) in the cellular structure of the sugar beet the solvent within the solid matrix is identical to the external extraction medium. To allow the sucrose to pass through the tissue, the semipermeability of the cell membranes must be overcome by thermal denaturation. The exit surface for the diffusion is enlarged by slicing the beet. The transport of substance by diffusion (release of sugar and entry of water into the cossettes) results from the concentration gradient existing in the cossettes. The driving force of diffusion appears to be not only the concentration gradient but rather the chemical potential. As the extraction proceeds, not only does this gradient diminish, but the concentration of nonsugar substances in the extract increases, so that the whole process must be stopped at the appropriate point. Extraction is counted as one of the major stages of manufacturing process in the sugar industry (Fig. 1).
